Superposed multi-junction color APS

ABSTRACT

A CMOS image sensor obtains color through the use of two or three superposed layers. Each pixel in the image sensor includes a plurality of superposed photosensitive p-n junctions with individual charge integration regions. The combination of each of the superposed layers provides increased sensitivity and resolution of a single chip color imager.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/522,286, filed Mar. 9, 2000, now U.S. Pat. No. 6,455,833, whichclaims the benefit of the U.S. provisional application Ser. No.60/124,084, filed Mar. 9, 1999.

TECHNICAL FIELD

This invention relates to image sensors, and more particularly to activepixel sensors having superposed regions.

BACKGROUND

CMOS image sensors have a significant advantage of allowing lower powerconsumption. An active pixel sensor (APS) is one example of a low powerconsumption image sensor which has photoreceptors, and buffer circuitry,and processing circuitry, all on one substrate.

Many different things can be done using the CMOS technology. Forexample, many of the applications by Photobit, Inc. of Pasadena, Calif.have enabled various operations to be carried out on the same substrateas the image sensor.

Certain resolutions are desired for different operations. For example,for a still camera, one often wants very high resolution, e.g. similarto the resolution that one could get from a photograph. This couldrequire more than 1½ megapixels. However, people are accustomed toobtaining less resolution in a video environment, which shows aprogression of information, e.g., 30 to 60 frames per second.

Another consideration is the way in which one obtains color from a colorsensor. Each pixel value includes an indication of values for red, greenand blue at the location of that pixel. However, in actuality, thesystem obtains red values from one pixel area, green from another, andblue from yet another. The three values are neighboring values, so theactually-obtained information is interpolated to obtain postulatedmagnitudes of colors at other locations.

Another way in which this can be done is by putting small prisms at eachpixel. A lot of adjustment can be required.

SUMMARY

The present invention obtains color in a CMOS image sensor with the useof two or three superposed layers. Each pixel in the image sensorincludes a plurality of superposed photosensitive p-n junctions withindividual charge integration regions. The combination of each of thesuperposed layers provides increased sensitivity and resolution of asingle chip color imager.

One aspect of the invention includes a photosensor comprising a firstcharge collection region having a first absorption length and a secondcharge collection region having a second absorption length. The firstcharge collection region and the second charge collection region aresuperposed. The photosensor further comprises a third charge collectionregion having a third absorption length. The third charge collectionregion is superposed with the first and second charge collection region.

Another aspect of the invention is a method of generating color in anactive pixel sensor comprising generating light of a first color in afirst charge collection region and generating light of a second color ina second charge collection region. The method further superposes thelight of the first color with the light of the second color.

DESCRIPTION OF DRAWINGS

These and other features and advantages of the invention will becomemore apparent upon reading the following detailed description and uponreference to th accompanying drawings.

FIG. 1 illustrates a color sensor with three superposed chargecollection regions.

FIG. 2 illustrates a pixel layout according to the present inventionusing a 4:2:2 sampling standard.

FIG. 3 is a cross-section and schematic diagram for three superposed p-njunction color APS according to the present invention.

DETAILED DESCRIPTION

During video signal processing, numerous data formats are used torepresent image information associated with each pixel of a video fieldso that an original image can be faithfully reproduced. For example, onecommon color format represents a color using red, green, and blue colorcomponents. With this color format, the color of each pixel isrepresented by quantities of red (R), green (G) and blue (B) colorcomponents detected in the original.

FIG. 1 illustrates a color sensor 100 with three superposed chargecollection regions 105. The charge collection regions 105 are superposedto provide the color sensor 100 with increased color sensitivity andresolution. The charge collection regions 105 include a first p-njunction 110, a second p-n junction 115, and a third p-n junction 120.Because the absorption length for incident photons in silicon iswavelength dependent, the charge collection regions 105 are highlysensitive to light of different color. The first p-n junction 110 issensitive to blue light, the second p-n junction 115 is sensitive togreen light, and the third p-n junction 120 is sensitive to red light.The spectral response of the charge collection regions 105 is dependentupon the thickness and location of the layers.

The superposed charge collection regions 105 may be used with a pixelhaving a 4:2:2 sampling mode. The 4:2:2 mode is a ratio of samplingfrequencies used to digitize the luminance (Y) and color differencecomponents (R-Y and B-Y). For example, the first color differencecomponent may represent the difference between the red image informationand the luminance image information (R-Y) and the second colordifference component may represent the difference between the blue imageinformation and the luminance image information (B-Y). The term 4:2:2denotes that for every four samples of Y, there are 2 samples each ofR-Y and B-Y, giving more chrominance bandwidth in relation to luminancecompared to standard 4:1:1 sampling.

FIG. 2 illustrates a pixel 200 layout according to the present inventionusing a 4:2:2 sampling mode. Applying color separation in an APS throughthe use of superposed regions is possible through development of pinnedand buried photodiodes, advances in color processing and the continuousscaling down of the CMOS features. The pixel 200 includes a greencomponent 205, a blue component 210, and a red component 215. Thecombination of the color components 205, 210, 215 provides for increasedcolor sensitivity in the pixel 200.

FIG. 3 is a cross-section and schematic diagram for three superposed p-njunction color APS 300 according to the present invention. The APS 300comprises a plurality of N+ floating diffusion regions 305, a pluralityof P+ floating diffusion regions 310, a P− buried region 315, a N−surface region 320, a fully depleted N− well 325, NMOS reset transistors330, 335, a PMOS reset transistor 340, a red output transistor 345, ablue output transistor 350, and a green output transistor 355. Each ofthe floating diffusion regions 305, 310 is connected to a resettransistor 330, 335, 340 and an output transistor 345 350, 355. The P+diffusion region 310 is connected to the PMOS reset transistor 340 andthe N+ diffusion region 305 is connected to the NMOS reset transistor330. The fully depleted N− well 325 overlaps the N+ diffusion region305. The N− well 325 also surrounds the P− buried region 315 and the N−surfaced region 320. The N− suffaced region 320 is proximate the P+diffusion region 310 and the P− buried region 315. Each of the floatingdiffusion regions 305, 310 preferably have different integration periodsthat allow each spectral selection to have flexible saturation exposure.

The color components of the pixel 300 are provided by the outputtransistors 345, 350, 355. In one embodiment, the output transistor 345outputs the red component, the output transistor 350 outputs the bluecomponent, and the output transistor 355 outputs the green component. Ifonly two output transistors are desired, the green component may beomitted.

The color APS 300 is capable of performing 4:4:4 sampling mode. In the4:4:4 sampling mode, there are always an equal number of samples ofluminance (Y) and color difference components (R-Y and B-Y). The 4:4:4sampling mode provides for more data to form the images, and thus thepotential of images having a higher resolution and clarity. To perform4:4:4 sampling, the color APS 300 preferably has at least two separatereset control lines. The number of reset control line is dependent uponthe number of implemented superposed layers, with each layer having aseparate reset control line.

Numerous variations and modifications of the invention will becomereadily apparent to those skilled in the art. Accordingly, the inventionmay be embodied in other specific forms without departing from itsspirit or essential characteristics.

1. A device comprising: a semiconductor substrate, having a firstsurface which is arranged to receive incoming irradiation; at leastfirst and second photoreceptor parts located in said semiconductorsubstrate, and arranged in said semiconductor substrate in a way suchthat charges produced by different wavelengths of said incomingradiation are respectively collected by said at least first and secondphotoreceptor parts; and at least first and second separately controlledreset transistors for respectively resetting said at least first andsecond photoreceptor parts.
 2. A device as in claim 1, wherein saidfirst and second photoreceptor parts are at different depths in thesemiconductor substrate.
 3. A device as in claim 1, wherein said firstand second photoreceptor parts are arranged in first and second areaswith different semiconductor characteristics.
 4. A device as in claim 1,further comprising a third photoreceptor part also arranged in saidsemiconductor substrate to collect charges produced by differentwavelengths of light than the charges collected by said first and secondphotoreceptor parts.
 5. A device as in claim 4, wherein said first,second and third photoreceptor parts each receive light of predominantprimary colors.
 6. A device as in claim 4, wherein said first, secondand third semiconductor parts are superimposed one over another.
 7. Adevice as in claim 4, wherein said first, second and third semiconductorparts are partially overlapping one another.
 8. A device as in claim 3,wherein one of said first and second areas comprises a P region, andanother of said first and second areas comprises an N region.
 9. Adevice as in claim 4, further comprising a third separately controlledreset transistor for resetting the third photoreceptor part.
 10. Adevice as in claim 1, further comprising first and second outputcircuits, respectively connected to said first and second photoreceptorparts, and producing output signals based on charges collected by saidfirst and second-photoreceptor parts, respectively.
 11. A device as inclaim 1, wherein said device is an active pixel sensor capable ofoperating in 4:4:4 sampling mode.
 12. An active pixel sensor comprising:a photoreceptor region formed in a substrate and comprising: a wellregion located within said substrate and doped to a first conductivitytype; a buried region located within said well region and doped to asecond conductivity type; and a surface region located within said wellregion near the surface of said substrate and doped to said firstconductivity type, said regions collecting charges associated withrespective wavelengths of applied light; first, second, and thirdstorage regions for respectively receiving and storing said chargesassociated with said respective wavelengths of applied light; and first,second, and third reset transistors for separately controlling the resetof said first, second, and third storage regions.
 13. An active pixelsensor as in claim 12, further comprising output circuits forselectively outputting signals from the first, second, and third storageregions representing the amount of charges stored in said storageregions.
 14. An active pixel sensor as in claim 13 wherein said outputcircuits comprise a first, second, and third output transistorassociated with said respective first, second, and third storageregions.
 15. An active pixel sensor as in claim 13, wherein said activepixel sensor is capable of operating in 4:4:4 sampling mode.
 16. Anactive pixel sensor as in claim 12, wherein said first conductivity typeis n-type and said second conductivity type is p-type.
 17. An activepixel sensor as in claim 16, wherein at least one of said first, second,and third reset transistors comprises an NMOS transistor, and at leastone of said first, second, and third reset transistors comprises a PMOStransistor.